I've never applied, but I know - not very well - someone who was in it recently, and what she said was very much what Caulk said. Surely no one, however naive, actually thinks that the twelve who end up on TV are the twelve best bakers who applied. The producers are casting, as they would for a sitcom, or a documentary where a variety of different experiences and faces are wanted. They consider your age, sex, ethnic background, and whether there's something unusual you can bring to the programme.
Besides being able to bake, obviously, I'm sure part of, say, Chetna's appeal was her interest in using Indian ingredients and recipes, just as Luis's was his design background and his Spanish-influenced dishes and flavours. Martha represents Youth, Diana Age, Norman Scots Presbyterian Plain Speaking, Richard Bluff Builder with an unexpectedly delicate touch, Jordan is Zany Guy etc etc.
Anyone interested in applying should read Danny the medic from last year's blog, Baking as Therapy. Without letting cats out of bags or being unpleasant about the programme, she talks about what it was like combining a demanding job with appearing, how the editing altered the real people she felt she knew, and why she wouldn't do it again.